With the development of radio and space technologies, several satellites based navigation systems (i.e. satellite positioning system or “SPS”) have already been built and more will be in use in the near future. SPS receivers, such as, for example, receivers using the Global Positioning System (“GPS”), also known as NAVSTAR, have become commonplace. Other examples of SPS systems include but are not limited to the United States (“U.S.”) Navy Navigation Satellite System (“NNSS”) (also known as TRANSIT), LORAN, Shoran, Decca, TACAN, NAVSTAR, the Russian counterpart to NAVSTAR known as the Global Navigation Satellite System (“GLONASS”) and any future Western European SPS such as the proposed “Galileo” program. The U.S. GPS system was built and is operated by the United States Department of Defense. The system uses twenty-four or more satellites orbiting the earth at an altitude of about 11,000 miles with a period of about twelve hours. These satellites are placed in six different orbits such that at any time a minimum of six satellites are visible at any location on the surface of the earth except in the polar region. Each satellite transmits a time and position signal referenced to an atomic clock. A typical GPS receiver locks onto this signal and extracts the data contained in it. Using signals from a sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time. In this application, we use the term Navigation Satellite System (NSS) to encompass any type of satellite-based communication system used for navigation, specifically terrestrial navigation, by a GPS receiver. The GPS receiver is typically included in a navigation device.
Navigation devices, including personal navigation devices (PND), such as those available from Garmin and other manufacturers, typically include extensive map databases covering entire countries or regions to provide real-time position displays and turn-by-turn directions among other things. In conventional systems, typically NSS data is passed to an application that utilizes a map database, and a map-matching algorithm determines which road is most likely to be the one where the navigation device containing the GPS receiver is traveling on. This is done so that the initial location calculated from the NSS data snaps to a physical geographical object, such as, a road, for a final output displayed by the navigation device. However, the navigation device seldom takes advantage of selective cartography information obtained by using the map-matching algorithm to further improve the positional accuracy of the navigation device by incorporating appropriate positional correction to the originally determined location derived from the NSS data.
Improving the positional accuracy becomes more of a necessity in environments where satellite signals are degraded, and, as a result, the GPS receiver frequently encounters problems in locking onto the signals that are needed for the calculation of position, velocity, altitude, and time. In a degraded signal environment (e.g., a signal environment where signal strength is below 28 dBHz), satellite signals can be weak or otherwise difficult for GPS receivers to lock on to. Degraded signal environments are often encountered in urban areas, such as cities with many tall buildings. A city with many tall buildings contain “urban canyons”, which are environments where streets cut through dense blocks of structures such as skyscrapers. In urban canyons, satellite signals are frequently not visible or are degraded due to the signals being partially or fully blocked by buildings, for example. Consequently, the problem of inaccurate position calculations by GPS receivers in degraded signal environments is especially acute in urban areas.
One way to improve the accuracy of a calculated GPS position is to make accuracy improvements with the aid of a map database. Some attempts have been made to provide cartography information from this map database back to the GPS receiver in real-time to aid in the receiver's navigation solution. For example, co-pending U.S. patent application Ser. No. 12/409,315, filed Mar. 23, 2009, titled, “Method and Apparatus for Improving GPS Positioning Using an Embedded Map Database,” describes cartography information embedded within a GPS receiver. However, embedding a map database in the GPS receiver itself may lead to bulkier navigation device size. In order to optimize the size of the navigation device, size of the map database itself and/or complexity of the map-matching or navigational algorithms may have to be compromised.
Accordingly, a method and apparatus for making better accuracy improvements to a GPS receiver's position calculations in degraded signal environments remain desirable, where selective cartography information can be extracted from a map database that is optimized for providing positional correction in tandem with a navigational routine executed by the GPS receiver. In other words, it is desirable to provide geographic aiding from a map database to improve a solution obtained by the GPS receiver from NSS data.